167. CALCULATION OF THE HYDROGEN DIFFUSION COEFFICIENT WITH ATOMIC TRAPPING IN ION BEAM DAMAGED TUNGSTEN
Name: Joseph Lincoln Barton
Grad Year: 2016
A 5 MeV Cu ion beam is used to induce controlled levels of damage, i.e. displacements per atom (dpa), in room temperature W samples to simulate neutron irradiation. The W samples were then exposed to a D plasma ion fluence of 10^24 ions/m^2 at 380 K, and the resulting D retention was measured using nuclear reaction analysis (NRA) with 3He ion beams and thermal desorption spectroscopy (TDS). Comparing the retention in pristine W (9x10^19 at/m^2) to retention in the damaged samples, retention increases 33% for 10^-3 dpa (1.2x10^20 at/m^2), 78% for 10^-2 dpa (1.6x10^20 at/m^2), and 340% for 10^-1 dpa (4x10^20 at/m^2). This indicates that the percent increase in retention is observed to be proportional to dpa^0.65. A simplified retention model has been proposed  that provides concentration profiles that can be directly compared to NRA data and total retention measurements. Taking the trapping energies from density functional theory (DFT) calculations, the only free-parameters are three defect densities of in-grain monovacancies, dislocations, and grain boundary vacancies. The model can fit D retention data in a pristine W sample within the experimental error of the measurements. The estimated intrinsic defect densities are then fixed in subsequent modeling of the influence of ion-beam damage. In this presentation, we show modeling results of the NRA retention profile after ion damage by increasing the monovacancy defect density proportional to the damage level. A simple extension of the model allows for the calculation of a new diffusion coefficient that is a function of the concentration of trapped atoms. The Frauenfelder diffusion coefficient is thought to be the value of diffusion without trapping effects, measured with temperatures in W above 1200 K. Below 1200 K, measurements of the diffusion coefficient are much lower than the Frauenfelder value . Numerical calculations of our diffusion coefficient show that large energy traps are responsible for the low values of diffusivity at lower temperatures. Differences in the measurements of Zakharov and Benamati are due to trapping energies greater than 1.65 eV. We quantified that saturated traps no longer affect diffusivity, allowing an increase towards the Frauenfelder value, and propose that measurement discrepancies are due to this phenomena. The increased density of defects after displacement damage inhibits diffusion further because of the additional trapping sites.  J.L. Barton, et al., J. Nucl. Mater. 463 (2015) 1129-1133.  R.A. Causey, J. Nucl. Mater. 300 (2002) 91-117.
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